Synaptic organization capable of feedback onto input neural processes is well known in the complex retinas of vertebrates, especially in the inner plexiform layer. The variety of neurons present, however, makes pharmacological results difficult to interpret. In the relatively simple retina of the dragonfly ocellus, two types of neurons are present, the receptor axons and the ocellar nerve dendrites which leave the retina and run to the brain. The finding of synapses from ccellar nerve dendrites back onto receptor axons provided a unique preparation for study of feedback interaction in good isolation from other neurons. Pharmocological experiments indicate that the input and feedback neurotransmitters at the receptor - ocellar nerve dendrite junction of this ocellus can be blocked selectively. Examination of the resulting intracellular response has made it possible to develop a model of synaptic feedback interaction in this simple retina. Additional experiments to develop and test this model in the ocellar preparation are being performed. Anatomical studies of ocellar nerve projections into the brain begun using cobalt staining of whole nerve continue through use of intracellular electrophoretic dye injection techniques in order to relate neural responses with specific neurons from the catalog of neurons recently identified. BIBLIOGRAPHIC REFERENCE: Patterson, J.A. and R.L. Chappell. 1976. Electrical activity and structure of receptor and second-order cells of the median ocellus of the dragonfly. Biol. Bull. 151,423. (Abstract).